1. Field of the Invention
The present invention relates to the field of piston ventilators for patients in need of respiratory assistance. More particularly, the invention is concerned with compensating for the pneumatic compliance of the ventilator including the compliance of the piston cylinder so that the prescribed tidal volume is delivered to the patient.
2. Description of the Prior Art
When a ventilator is operating in a volume control mode, a prescribed tidal volume is specified for delivery to the patient. If only the prescribed tidal volume is added to the patient circuit, the patient will receive less than this tidal volume because of the pneumatic compliance of the system. That is, as a gas volume is added, the pressure in the circuit increases and the volume delivered to the patient is less.
In order to compensate for the pneumatic compliance, prior art ventilators determine, after the completion of a breath, the shortfall in the delivered volume compared to the prescribed tidal volume. This shortfall is then added to the input volume on the next patient inspiration. However, because of pneumatic compliance, the added amount may be insufficient to ensure delivery of the prescribed volume. The shortfall is again determined and a new additional volume added during the next inspiration. As many as eight iterations over eight breaths may be required before the prescribed tidal volume is actually delivered to the patient.
The prior art problem of compensating for pneumatic compliance is more acute with piston ventilators. This is because the piston can end up in a different position within the cylinder at the end of each breath. In other words, the system compliance is not static, but rather, changes with each breath. As a result, prior art piston ventilators have been unable to provide the prescribed tidal volume with the desired level of precision and reliability.